The subject matter disclosed herein relates to a method of testing a passive optical element, such as an optical fiber or an optical coupler.
Operators of data communication networks employing optical fiber cables require that the optical fibers forming the cables meet very strict requirements regarding change in optical power loss through the fibers under various mechanical stresses. The mechanical stresses may be applied by direct mechanical action, such as bending, twisting and crushing, or result from environmental effects, such as change in temperature. Testing to determine change in optical power loss of an optical fiber under change in mechanical stress due to environmental effects is referred to herein as enviro-mechanical testing whereas testing to determine change in optical power loss of an optical fiber under change in mechanical stress due to direct mechanical action is referred to herein as direct mechanical testing.
The current procedure for enviro-mechanical testing of optical fiber involves monitoring loss at one or more critical transmission wavelengths using a light source and power meter, applying a stress, and measuring the change in optical power loss resulting from the applied stress against relevant industry and/or end-user specifications. FIG. 1 shows one typical arrangement for performing a test to measure change in power loss. An optical fiber under test (FUT) 8 is placed in a suitable test fixture 10, which comprises an environmental stress chamber in which a controlled environmental stress may be applied to the FUT. The FUT has fiber pigtails that extend to the exterior of the stress chamber and are connected by fusion splices 12, jumpers 14, and optical connectors 16 to, respectively, a light source 18 and a power meter 20, which also are located outside the stress chamber.
The light source/power meter (LSPM) approach to measuring changes in loss is subject to disadvantage. For example, the real changes in loss of the FUT might be obscured by drift in the power output of the light source or drift in the response of the power meter over the duration of the test, and there may also be drift in loss of the optical connectors 16.
The test arrangement shown in FIG. 1 may be adapted to test several fibers substantially concurrently, such as a test sample of fibers in a multi-fiber cable, each connected to its own pair of jumpers 14, by providing optical switches 22 (shown in dashed lines) between the jumpers 14 and, respectively, the light source and power meter and controlling the switches 22 to select the fibers in turn. However, optical switches introduce another source of drift in power loss in the test channel (the optical path between the light source and the power meter).
Although drift in the source power and detector response can be monitored using a reference channel, the reference channel and the FUT cannot be monitored simultaneously. Further, in order to utilize a reference channel it would generally be necessary to interpose an optical switch between each jumper 14 and the adjacent optical connector 16 and drift in loss in the optical switches may impair the accuracy with which the change in power loss of the FUT can be measured.
Another arrangement that may be used for enviro-mechanical testing of an optical fiber is shown in FIG. 2. The fiber under test (FUT) has an upstream end, which is outside the stress chamber and is connected by a fusion splice 12 to the downstream end of a buffer fiber 26. The upstream end of the buffer fiber 26 is connected through a optical connector 30 to the port of an optical time domain reflectometer (OTDR) 34. The terms “upstream” and “downstream” are used herein relative to the direction of propagation of light from the port of the OTDR towards the FUT. The downstream end of the FUT is connected by a fusion splice 12 to the upstream end of a second buffer fiber 36.
In operation, the OTDR 34 acquires a data set that can be represented graphically as a trace showing power loss through the test channel (the optical path into which light is launched by the OTDR, and from which return light is received by the OTDR) as a function of distance. This trace, commonly referred to as a signature, may have the appearance shown in FIG. 3. Each segment of the signature corresponds to a segment of the test channel. In FIG. 3, the peak 40 originates from reflection in the optical connector 30, the peak 42 originates from a reflection at the glass-air interface at the far end of the buffer fiber 36, the substantially linear segments 44, 46 represent the power of return light received from the buffer fibers, and the steeper substantially linear segments 48, 50 represent power of return light received from the fusion splices. The operator places markers 52, 54 on the FUT and the OTDR measures the power loss in the segment between the markers. The measurements are not affected by loss drift in the elements, such as connectors and switches, outside the segment of the test channel that is between the markers. However, noise on the OTDR signature makes a two-point loss measurement of this type inherently noisy, injecting uncertainty into the measurement and possibly obscuring real changes in power loss.
Direct mechanical testing may be performed using an equipment arrangement that is schematically similar to that shown in FIG. 2. In the case of direct mechanical testing, however, the test fixture 10 applies stress to the FUT by direct mechanical action. Generally, the power loss is measured before the stress is applied and after the stress has been removed for a sufficient time to allow the FUT to recover.
Current standards for power loss change require a maximum loss change for 90% of the fibers in a fiber optic cable when placed under a specified stress of no greater than 0.05 dB. Current methods for measuring loss change have a precision no better than +/−0.05 dB. It is desirable that the minimum measurement error should be substantially less than the maximum permitted loss.